This invention relates to apparatus capable of performing a discrete cosine transform with light-weight, low-cost, high-speed hardware suitable for real-time television image processing.
The cosine transform is suitable for real-time television image processing, specifically for obtaining an acceptable picture when the number of bits of information available for describing the picture and/or the channel bandwidth are severely limited.
The number of bits of information in any system are always limited due to the economics of the situation. The greater the number of bits in a system the greater the cost. Or, given a certain number of bits, it is economically advantageous to have as many useable channels as possible with that given number. With the apparatus of this invention, even when the number of bits are reduced by a ratio of twelve to one, a satisfactory picture is transmitted. This means that twelve different channels, transmitting a picture having the quality of this one invention, can be transmitted with the same number of bits of information as only one channel having no data compression, as in the prior art.
Theoretical work and simulation studies have shown that the discrete cosine transform is nearly optimum for image redundancy reduction. This is discussed by Means, R. W., H. J. Whitehouse, and J. M. Speiser, REAL TIME TV IMAGE REDUNDANCY REDUCTION USING TRANSFORM TECHNIQUES, in "New Directions in Signal Processing in Communication and Control", NATO Advanced Study Institutes Series, Noordhoof-Leyden, 1975. The discrete cosine transform may be interpreted as a discrete Fourier transform of a symmetrized version of the image data block. Prior art means for performing the discrete Fourier transform, such as Fast Fourier Transform (FFT) hardware or chirp-z transform hardware may also be used to perform the discrete cosine transform. The chirp-z transform (CZT) devices are to be preferred to the FFT devices since the data block size is not restricted to be a highly composite number for the CZT, and also the CZT is about log.sub.2 N times faster (where N is the transform block length), using components with the same operation rate. However, the size of the transform block for the CZT is limited by the number of independent taps in the transversal filter. A filter length of at least 2 N-1 taps has previously been required to implement a discrete cosine transform of an N-point data block. This invention implements a discrete cosine transform of length N using only filters with N taps, thus either reducing the filter length required or permitting a longer block to be transformed with filters of a given length.
One of the principle advantages of this invention is the ability to perform a discrete cosine transform on longer blocks with filters having a given number of taps. This will permit a higher picture quality for a given transmission bandwidth, or a reduced transmission bandwidth for a given picture quality requirement. This invention replaces a long time-invariant transversal filter by a shorter time-variant transversal filter, where the latter is composed of two (complex) short time-invariant transversal filters, together with multipliers, signal generators, and a switch.
The transversal filters of this invention may be acoustic surface wave tapped delay lines, charge transfer tapped delay lines, or other tapped delay lines, or digital correlators. Similarly, the function generators which provide the discrete chirps may be read-only memories, acoustic surface wave filters, charge transfer devices, or digital shift registers. The function generators which provide the sequences of .+-.1's may be any of the aforementioned or may be digital switching circuits.